1. Field of the Invention
This invention relates to a process for producing tert-butanol by selective hydration of isobutylene in a hydrocarbon mixture comprising isobutylene and n-butene.
2. Description of the Prior Art
Tert-butanol is useful as an industrial starting material for preparing a variety of products. For example, tert-butanol is employed as a starting material for preparing methacrylonitrile or methacrolein which is an intermediate for preparing methyl methacrylate, and methacrylonitrile or methacrolein cannot be obtained from sec-butanol originating from the hydration of n-butene.
Heretofore, in selectively hydrating the isobutylene in a mixture of isobutylene and n-butene, a 50 to 65% sulfuric acid or hydrochloric acid, a hydrochloric acid solution of a metal chloride, a strongly acidic ion-exchange resin or a solid acid is employed as the catalyst. However, according to the method of using sulfuric acid, the dimer, trimer and polymers of isobutylene are by-produced and part of n-butene, especially 1-butene tends to be hydrated to give sec-butanol, and furthermore, the strong corrosion of sulfuric acid requires expensive corrosion-resistant materials, making an industrial plant uneconomical. The method of using an inorganic acid such as hydrochloric acid or a halide such as tin chloride disadvantageously by-produces tert-butyl halides and also has the problem of the material of the plant due to their remarkable corrosion.
In order to avoid these disadvantages there is provided a method of using a strongly acidic ion-exchange resin or an insoluble solid acid, but their hydration activity is low and generally a high hydration temperature ranging from 120.degree. C. to 200.degree. C. is required. Moreover, at this temperature the life of the ion-exchange resin is short and filtration of finely pulverized particles from a fluidized liquid is very difficult and as a result, the proces becomes complicated. Fundamentally, with increased temperatures the conversion of isobutylene to tert-butanol decreases more from the standpoint of chemical equilibrium. Accordingly, the concentration of tert-butanol in an aqueous solution at equilibrium becomes low when the catalyst requiring high temperatures is used. As a result, the reaction apparatus unfavorably becomes large from the industrial viewpoint.
Generally, it is known that an aqueous solution of a heteropolyacid such as tungstosilicic acid, molybdophosphoric acid and tungstophosphoric acid can be employed in hydrating olefins. However, as far as an olefin mixture of n-butylene and isobutylene is concerned, there has not been known such a catalyst having a high activity even under mild reaction conditions that can selectively convert isobutylene into tert-butanol without forming polymers such as the dimer and the trimer of isobutylene and with n-butenes unreacted while maintaining the initial catalytic activity for a long period of time and that makes possible use of an ordinary industrial material, for example, stainless steel as the construction material.
More specifically, British Pat. No. 1,281,120, Japanese Patent Publication Nos. 35052/1975 and 35053/1975 describe the hydroxylation of a 1:1 mixture of isobutylene and n-butene in the presence of a heteropoly-acid ion. With taking into account the drawbacks of corrosion of the material of the reactor and the side reactions, the hydration is conducted at a high temperature and a high pressure (200.degree. C., 250 Kg/cm.sup.2) while maintaining the concentration of the heteropoly-acid low and the pH of the reaction high, resulting in a 1:1 mixture of sec-butanol and tert-butanol as the product. In this case, in order to complete the reaction, several hours are necessary due to the low catalytic activity. Moreover, the selective hydration of isobutylene to tert-butanol in the presence of n-butene cannot be achieved.
Further, Japanese Patent Application (OPI) No. 13711/1976 discloses that when the hydration of isobutylene is conducted at a concentration of the heteropoly-acid as high as 10 to 70% by weight at a temperature of from 100.degree. C. to 170.degree. C., corrosion of the material of reactors and by-production of olefin polymers can be prevented and the life and the hydration activity of the catalyst are satisfactory. However, there is no disclosure on the selective hydration of isobutylene in mixed butylenes.
Also, in the reaction system using the conventional catalysts, the tert-butanol produced is substantially present in the aqueous liquid phase, and accordingly, after the removal of the remaining hydrocarbon mixture, for example, by separation of the organic liquid phase from the aqueous liquid phase, tert-butanol is recovered from the aqueous liquid phase by any conventional method such as distillation under reduced pressure and salting-out.
However, in recovering tert-butanol from the aqueous liquid phase by distillation, polymers are produced and a large quantity of heat is necessary. The distillate contains more water than the azeotropic mixture of water and tert-butanol and in general, it is impossible to increase the concentration of tert-butanol from a range of 80% to 89%. In the salting-out method, removal of the salt is necessary for re-using the acid catalyst contained but this is very difficult and industrially impractical.
Japanese Patent Application No. 4165/1972 discloses a process for hydrating isobutylene in the presence of an acid catalyst and an inorganic acid salt in order to make the tert-butanol produced the upper phase. However, this method employs sulfuric acid as the catalyst and the hydrocarbons such as n-butene are not liquefied. As a result, the tert-butanol produced contains a large amount of water as clearly understood from the Example of this prior art and cannot be obtained at a high concentration. Further, the presence of sulfuric acid causes strong corrosion of the material of the reactor and a large amount of diisobutylene is by-produced and thus, this method is industrially of little value.